- With the rack in the lowest position, preheat the oven to 425°F (220°C). Line a baking sheet with parchment paper.
- In a small bowl, combine the sugar and cardamom. Set aside.
- On a lightly floured surface, roll out the puff pastry into a 10-inch (25 cm) square. Prick the surface using a fork. Cut the dough into 2 rectangles and place them on the baking sheet.
- Sprinkle the dough with half of the sugar mixture. Spread the grape slices overtop and sprinkle with the remaining sugar mixture.
- Bake for 25 minutes or until the pastry is golden brown. Let cool completely.